Acid mine drainage (AMD) results from oxidation of metal sulfide minerals, primarily pyrites and other sulfide ores. The acidic reaction products are absorbed by the descending waters and rising subsurface waters which enter the surface water ecosystems. Some large mine sites currently generate an excess of six million gallons AMD per day. One particular mine site, located in Northern California, generates 25% of the total metal contamination entering the ground water of the entire United States.
U.S. Pat. No. 4,695,378 discloses treatment of AMD according to the following steps:
(1) neutralization PA0 (2) aeration PA0 (3) settling and disposal of sludge PA0 (4) effluent discharge
Neutralization, aeration, and settling equipments are expensive and require large structures and excavation for large treatment facilities.
U.S. Pat. No. 3,511,777 discloses raising the pH from the acid range to the basic range by mixing with lime. The cation constituents combine with the calcium carbonate to generate bicarbonates. The sulfate ion SO.sub.4.sup.-- ion remains in large concentrations in the treated water.
Another widely practiced method used for removing metals from acid water is a pH control method in which CaOH is added to the waste stream to raise the pH. With single valency metal contaminants, most of the metal can be removed by raising the pH of an initially highly acidic solution to 8.5. With high valency ions, the pH must be raised to above 10.5. The sludge generated in some of these cases has required the use of separators in place of the more economical filters.
The major problem encountered with hydroxide precipitation processes with multiple metal contaminants is the wide range of solubilities of the formed hydroxide precipitates. In order to precipitate most of the metal, the pH must be raised to the range 10.5 to 11.0. When the basic solution is neutralized, some of the metal goes back into solution and recontaminates the water.
In acid industrial waste water, the heavy metal ions are usually singly charge. (A notable exception is the effluents from electroplating processes.) Natural contaminated water typically contains several ionic states of the same metal. Each ionic state, when combined with a neutralizing compound containing OH, form as metal hydroxides of varying stoichiometries. Some of these hydroxides are insoluble precipitates. Most of the generated hydroxides are characterized by a strongly pH dependent solubility. These soluble hydroxides in some cases can be partially removed by physical absorption or crystal chemical inclusion (chemisorption) and may be lowered to acceptable levels.
The use of sodium, potassium and calcium hydroxides creates a metal bearing sludge that is very difficult to filter effectively and the metal hydroxide cake is hopelessly cross contaminated so as to be beyond economical separation and are classified as hazardous waste with all the problems and expense of hazardous waste disposal. Treatments of waste water containing large concentrations of sulfate ions produce water having a large concentration of SO.sub.4.sup.-- even though the pH is in an otherwise acceptable range.
The Iron Mountain Mine Site located near Redding, Ca. and the Berkeley Pit located at Butte Mont. are particularly notorious examples of the undesirable environmental impact of AMD. At the Iron Mountain Site, there are fifteen or more highly toxic contaminants present, some in large quantities. The AMD waste water from the Iron Mountain waste water in California has a pH between 0.58 to 0.75.
Table I lists the average concentration over a twelve month period in the Iron Mountain Mine Site.
TABLE I ______________________________________ Aluminum 2300 ppm Arsenic 33.5 ppm Barium &lt;10.0 ppm Beryllium &lt;0.5 ppm Cadmium 10.7 ppm Chrome &lt;0.1 ppm Copper 350 ppm Iron 13.1 ppm Lead 3.5 ppm Magnesium 605 ppm Nickel &lt;4 ppm Thallium &lt;0.2 ppm Vanadium &lt;0.2 ppm Zinc 1,595 ppm SO.sub.4 55,200 mg/liter Total dissolved solids 81,565 ppm mg/liter. ______________________________________
TABLE II lists the impurity content of a sample taken from the surface level of the Berkeley Pit.
TABLE II ______________________________________ Ca 463 ppm Mg 452 ppm Pb 0.048 ppm SiO.sub.3 103.0 ppm Fe 373 ppm Mn 233 ppm Al 79.9 ppm Ag &lt;79.9 ppm B &lt;0.10 ppm Cd 2300 ppm Cu 189 ppm Li 0.272 ppm Mo &lt;0.040 ppm Ni 1250 ppm Sr 1700 ppm As 0.103 ppm Co 1440 ppm Cr 0.041 ppm SO.sub.4 &gt; 6930 mg/liter ______________________________________